Tree-Derived Dissolved Organic Matter (Tree-DOM) in Throughfall and Stemflow
2. Materials and Methods 1. Study Site Description
The study was conducted in a forest on Skidaway Island, Georgia, USA, located at 31.9885◦ N, 81.0212◦ W (Figure 1a) along Georgia’s coast. Climate is humid subtropical (KöppenCfa).
Thirty-year mean annual temperature and precipitation (exclusively rainfall) was 19.3 ◦C and
58
975 mm [22]. The forest site (Figure1b) contained 162 stems ha−1ofJuniperus virginianaL. (eastern red cedar, hereafter “cedar”). Cedars on site host substantial epiphyte biomass,Tillandsia usneoidesL.
(Spanish moss) (Figure1c), yet some individual trees were bare. Further site information and a complete site inventory can be found in [11].
Figure 1.Location of (a) Skidaway Island in Chatham County (shaded area), Georgia, USA; and (b) the study site location on the Skidaway Institute of Oceanography campus; (c) Photograph provided showing an example cedar canopy hosting the epiphyte,Tillandsia usneoidesL.
2.2. Rainfall, Throughfall and Stemflow Sampling
Sampling of rainfall, throughfall and stemflow was performed for five storms during the 2017 growing season: 14 May, 3 June, 30 July, 28 August and 23 October following published methods [11].
Three bulk rainfall and twenty throughfall samplers consisting of 0.18 m2high density polyethylene (HDPE) bins were deployed immediately before each storm. Rainfall was collected in an open area immediately beside the forest site and throughfall samplers were evenly split between bare (n= 10) and epiphyte-covered (n= 10) cedar canopies. Ten individual cedar trees were selected for stemflow sampling, five each with bare and epiphyte-covered canopies (individual tree characteristics provided in Table S1). Stemflow samplers consisted of collars made from polyethylene tubing cut longitudinally, wrapped around the stem at 1.4 m height, fixed to the stem with aluminum nails, sealed to the bark with silicone, and connected to a 120 L HDPE bin. All samplers were pre-cleaned with pH 2 (using trace clean 6 N HCl) ultrapurified water (Milli-Q), then triple-rinsed with ultrapure water, air dried, and covered until the start of a storm. Sample volumes were measured manually with graduated cylinders. Samples for DOC quantification were collected within hours after a storm, filtered to 0.2μm through hydrophilic polypropylene syringe filters (Acrodisc) into precombusted glass vials, acidified, then stored at 4◦C in the dark until quantification of DOC concentration (hold period no longer than one month). All sampling materials were precleaned with acidified ultrapure water, triple-rinsed with ultrapure water, then triple-rinsed with sample water.
2.3. Bioincubations
Bioincubations lasting 14 days and based on a modified protocol [7,23] were performed to estimate the biolabile portion and decay coefficients for tree-DOM in throughfall and stemflow for each storm. Immediately after each storm, in addition to the sample taken for DOC analysis in Section2.2, four 20 mL samples per stemflow sampler and from a composite sample of every two throughfall samplers were filtered to 0.2μm and placed into precombusted glass vials. A bacterial inoculum was added (2 mL) to each vial along with 2 mL of Nitrogen-Phosphorous-Potassium (NPK) nutrient solution (10:10:10) to prevent nutrient limitation from constraining biodegradation. As throughfall and stemflow at this site contain 104–106bacteria mL−1[24], the inoculum was prepared for each storm
Forests2018,9, 236
from a volume-weighted composite of freshly collected throughfall and stemflow samples filtered through a 50μm mesh to remove microbial grazers and coarse particulates. Caps were placed loosely on the bottles to allow air movement, then samples were incubated for 1, 2, 4, and 14 days at 25◦C in the dark on a shaker table (60 rpm). After bioincubation, each sample was filtered to 0.2μm into a new precombusted glass vial, acidified, then stored at 4◦C in the dark until quantification of DOC concentration (hold period no longer than one month).
2.4. Dissolved Organic Carbon Concentrations
Concentrations of DOC were determined as nonpurgable organic C using a total organic carbon (TOC)-VCPH analyzer with an ASI-V autosampler (Shimadzu, Columbia, MD, USA). Calibration curves were made with potassium hydrogen phthalate stock solution.
Instrument reproducibility was checked against deep seawater reference material from the Consensus Reference Material (CRM) Project. CRM analyses were <5% from reported (http://yyy.rsmas.miami.
edu/groups/biogeochem/Table1.htm). This configuration has a minimum DOC detection limit of 0.034±0.004 mg L−1with typical standard errors for DOC concentration being 1.7±0.5% [25].
2.5. Data Analysis
First-order decay curves were fit to the DOC concentrations quantified after bioincubations:
%DOC remaining=be−kt+c (1)
wherebis the biolabile proportion,kis the decay coefficient, andcis the recalcitrant proportion of tree-DOM that would theoretically resist biodegradation indefinitely under these experimental conditions. First-order decay curves were only fit to sample data where DOC concentrations stopped decreasing between two consecutive measurements. Tree-BDOM yield (mg-C m−2mm−1rainfall) for each storm from each flux/cover type was computed as the product ofband total tree-DOM yield.
Total tree-DOM yield was calculated as DOC concentration (mg-C L−1)×water volume (L)/canopy area (m2) and rainfall amount (mm). Two-way Analysis of Variance (ANOVA) with a Tukey’s Honest Significant Differences (HSD) test was performed to compare initial DOC concentrations between fluxes with and without epiphyte cover using Statistica 13.2 (Statsoft, Tulsa, OK, USA). The threshold for significance wasp< 0.05 unless otherwise noted and variability about the mean is expressed in the text as standard deviation.
3. Results
3.1. Hydrometeorology for Sampled Storms
Sampled storms ranged in magnitude from 8 mm to almost 50 mm, while storm duration ranged from 8.5 to 59.3 h (Table1). Rainfall intensity for sampled storms varied from 0.7 to 1.8 mm h−1 (Table1). Throughfall volumes were generally larger beneath bare compared to epiphyte-covered cedar canopies, except for the largest storm (14 May 2017; Table1). Stemflow volumes from the sampled bare cedar trees were 2–5 times greater than observed beneath the epiphyte-covered cedars (Table1).
The sum of throughfall and stemflow exceeded total rainfall for the 30 July storm (9.2 mm net rainfall versus 8.9 mm gross rainfall), which is a common artifact observed when throughfall drip points are oversampled [26], rainfall is undersampled, or wind conditions permit greater three-dimensional rainfall capture area than represented by two-dimensional projected canopy areas [27].
60
Table 1.Rainfall conditions and throughfall and stemflow (mm across canopy area) for the five storms sampled for biolability testing. Throughfall and stemflow as percent rainfall provided in parentheses.
Condition 14 May 2017 3 June 2017 30 July 2017 28 August 2017 23 October 2017
Magnitude, mm 48.3 8 8.9 26.1 30.8
Duration, h 59.3 11.5 8.5 30 17.3
Intensity, mm h−1 0.8 0.7 1.0 0.9 1.8
Throughfall
Bare, mm (%) 26.4 (55%) 4.7 (58%) 7.5 (84%) 19.0 (73%) 25 (81%)
Epiphyte, mm (%) 28.0 (58%) 2.1 (26%) 2.9 (33%) 13.8 (53%) 20.0 (65%) Stemflow
Bare, mm (%) 8.0 (17%) 1.0 (12%) 1.7 (19%) 3.9 (15%) 4.4 (14%)
Epiphyte, mm (%) 4.0 (8%) 0.2 (3%) 0.4 (4%) 0.9 (4%) 1.6 (5%)
3.2. Initial DOM Concentrations
DOC concentrations in rainwater (<7 mg-C L−1) were always significantly lower than in throughfall and stemflow (entire dataset provided in Table S2). The highest mean concentration across sample types was found in stemflow from epiphyte-covered canopies, 85 ± 38 mg-C L−1, followed by epiphyte-covered throughfall, 64±34 mg-C L−1, then bare-canopy stemflow, 55±34 mg-C L−1, and bare-canopy throughfall, 35±19 mg-C L−1(Table S2). The maximum initial tree-DOM concentration observed was 143 mg-C L−1 from epiphyte-covered stemflow (Table S2). Significant differences between sample types were found for bare-canopy throughfall versus epiphyte-covered throughfall, bare-canopy stemflow versus epiphyte-covered stemflow, and bare-canopy throughfall versus epiphyte-covered stemflow (p< 0.001).
3.3. Interstorm Tree-DOM Biolability
Tree-DOM concentrations declined over the bioincubation experiments, conforming to first-order decay models (Figure S1 and Table S3). Greater tree-BDOM proportions generally related to larger decay coefficients (Figure2). Mean tree-BDOM proportion was greatest during the smallest magnitude storm (3 June, 8.0 mm) for all sample types except bare-canopy throughfall, 70±20% (Figure2).
Eighty-eight percent of tree-DOM, on average, in stemflow from bare and epiphyte-covered canopies and epiphyte-covered throughfall was biolabile for the 3 June storm (Figure2). The 3 June storm also had the largest range in mean decay coefficients across sample types, 2.4–6.7 day−1for bare-canopy versus epiphyte-covered throughfall, respectively (Figure2). The proceeding storm, 30 July, had the largest range in mean tree-BDOM across sample types, 34±12% for epiphyte-covered stemflow versus 73±9% for bare-canopy throughfall, but the mean decay coefficient was generally similar (Figure2). For the 28 August storm, all sample types had similar mean tree-BDOM proportions, 49–55%, but decay coefficients were as low as 1.4 day−1for epiphyte-covered stemflow and over double this value, 3.7 day−1for bare-canopy throughfall (Figure2). The 23 October and 14 May storms were both large magnitude storms (30.8 and 48.3 mm, respectively), and tree-BDOM proportions were generally the lowest on average, barring epiphyte-covered throughfall on 23 October (Figure2). Tree-BDOM regardless of epiphyte cover or flux type produced the lowest decay coefficients, 0.2–0.8 day−1, for the largest, 14 May, storm (Figure2).
Forests2018,9, 236
Figure 2.Mean, standard error, and standard deviation for biolabile proportion (black) and the decay coefficient (red) across the five sampled storms during 2017 arranged by increasing storm magnitude for bare-canopy throughfall (TF-B), epiphyte-covered throughfall (TF-E), bare-canopy stemflow (SF-B), and epiphyte-covered stemflow (SF-E).
3.4. Tree-BDOM Yield
For individual storms, mean tree-BDOM yield from any cover ranged over an order of magnitude, from 3.1 to 62.6 mg-C m−2mm−1of rainfall (Table2). Greater water yields of throughfall from bare canopies (Table S2) compared to all other fluxes resulted in generally larger tree-BDOM yields for smaller storms (3 June, 30 July: Table1) compared to throughfall from epiphyte-covered canopies (Table2). Under rain events large enough to saturate the epiphyte-covered canopy areas (14 May, 28 August, 23 October), throughfall yielded greater tree-BDOM beneath epiphyte-covered canopy than bare canopy (Table2). For stemflow, tree-BDOM yields were consistently greater from bare compared to epiphyte-covered canopies for all storms (Table2) regardless of storm conditions (Table1) and despite higher initial DOM concentrations from epiphyte-covered canopies (Table S2).
Table 2.Mean and standard deviation of biolabile tree-DOM (tree-BDOM) yields for throughfall (TF) and stemflow (SF) samples in each storm.
Biolabile Yield (mg-C m−2mm−1Rainfall)
14 May 2017 3 June 2017 30 July 2017 28 August 2017 23 October 2017 TF, bare 10.3±2.0 33.3±15.2 62.6±32.5 18.9±6.1 16.3±3.5 TF, epiphyte 12.2±1.5 32.8±18.8 25.5±10.1 45.0±17.1 28.6±6.6
SF, bare 5.6±1.8 10.6±5.5 16.1±8.7 7.4±2.5 4.9±1.1
SF, epiphyte 3.9±2.2 3.7±1.9 5.4±3.0 3.3±1.7 3.1±0.6
4. Discussion